McKellar, 34, confirmed to People magazine Wednesday that she and husband Mike Verta will welcome the baby this fall.
"I'm nearing the end of my first trimester and I'm so excited - I've been bursting to tell people," she said.
She and Verta, 36, married in March 2009.
McKellar, who went on to study mathematics at UCLA after her time on the "Wonder Years," is the author of "Math Doesn't Suck: How to Survive Middle-School Math without Losing Your Mind or Breaking a Nail" and "Kiss My Math: Showing Pre-Algebra Who's Boss." Her third book, "Hot X: Algebra Exposed," comes out in August.
Actress Who Played Winnie Cooper Expecting First Her Child with Husband Mike Verta